You have a relatively low resolution, and your stated requirement is that "mid" is acceptable...considering that your PSU becomes extremely inefficient at over 80% load, you would do quite well with a HD6770 or GTX550ti, either of which should be able to handle high settings at that resolution.If you are interesting in upgrading your monitor to a higher resolution, and also have the budget to replace your PSU with a quality model, please post your total budget to get the most useful suggestions.

i dont want to upgrae my psu i thionk its good one i just want to upgrade my GPU my budget is about 70 pounds for gpu yes and i will OC my CPu to 3.00 Ghz but i dont know how and i dont know i f i could with my mobo

That brings me to another point.Since your mobo is a little old,which likely means it's a 1.0 slot,I would avoid 2.1 cards.Not because of a bottleneck but because of compatiblity issues.Technically they are backwards compatible but you have to have a BIOS update in order to use a 2.1 card.Trust me just stick with a 2.0 card to avoid any headaches.

At stock speeds of 2.5ghz you will bottleneck them but it should be minimal.O.C.ing will only increase the performance and lessen the bottleneck.It should be pretty easy to get to 3ghz or 3.2ghz.Just make sure you read that guide and it will walk you through it step by step.